I have both Serenity and Firefly, and indeed Cortex classic was not all that notable of a system. Cortex Plus however was an almost entirely different animal, revving into a more narrative-type system that has evolved even further into Cortex Prime (which now occupies one of the top spots for my go-to/favourite systems to run/play in). Also, while I never played it, I would say that the Cortex Plus Marvel Super Heroes probably also worked well, and I've heard that the Cortex powered Smallville was excellent.Not a revision, a total rewrite from scratch... and very worthy. But getting hard to find, and the electronic editions pulled by MWP, despite the approach of Fox to continue to extract income from old games post-license until someone else wants the license... Buffy, Angel, and Army of Darkness are still for sale at DTRPG, even tho' the licenses ended a decade ago.
That said, I too will crown WEG's Star Wars as the best licensed RPG I've played thus far. The system (especially at the time) and the sourcebooks combined well to create a rich playing field and some very SWian action/adventure moments.